Using pulse burst saturation recovery 19F NMR EPI relaxometry we are able to achieve high spatial and temporal resolution to measure tumor oxygenation on the basis of spin lattice relaxation of hexafluorobenzene. Several trends in tumor oxygenation are emerging: those tumor regions which are well oxygenated exhibit large dynamic changes in pO2 when inspired gas is altered e.g. from 33 to 100% O2. In contrast relatively hypoxic regions show very little change. However, individual tumor subregions behave heterogeneously, emphasizing the need to perform measurements on individual tumors. The small increases in the hypoxic regions are probably the most significant since it is the hypoxic clonogenic cells which defy therapy. A small increase in oxygenation may significantly improve prognosis for radiotherapy. We have now examined the influence on irradiation on an AT1 tumor and the results showed specific differences in tumor pO2 dynamics. We have found that hexafluorobenzene clears from the tumors with a half life of approximately 6 hours. This is sufficiently slow as not to interfere with relaxation measurements and provides the exciting possibility of simultaneously measuring long term tumor blood flow.